Yes, you read that right. Don’t overdo healthy foods. Let me explain: You shouldn’t overdo food, period, and healthy food is, well, food.
Over the last few weeks, I’ve repeatedly come across articles, blog posts and forum posts that ask essentially the same question: “Can healthy food make you fat?” Many of these questions were directed at specific healthy foods (I won’t name names), but I noticed a certain generality, as well.
Here’s the problem: When you decide that you need more of something in your diet (fiber, potassium, iron, protein, vitamin B6, or whatnot), and you start adding foods to your meals and snacks that are rich in that certain something, and you don’t eliminate something else from your diet, guess what? You’re suddenly eating more calories! Soon, you’re shopping for bigger pants and you don’t understand why, because you “eat so healthy.”
Case in point: I eat two or three Brazil nuts every day because they are rich in the mineral selenium. Selenium is vital for many metabolic processes, including helping the thyroid make hormones. My thyroid hormone levels were not quite optimal when I had them measured last year, so I started bumping up my intake of a few key foods to help support thyroid function (seaweed is another, for the iodin). But when I eat those Brazil nuts, I count them as part of a planned snack. They are not an extra.
You might now wonder, “Well, why should I even try to get nutrients from food if it just makes me eat too many calories and put on weight?” That’s a good question. My answer is twofold. 
First, nutrients from food are more easily assimilated by your body, in part because they are part of a complete package with other nutrients (some identified, some not) that are often needed for the main nutrient to actually do anything. 
Second, in some cases, you will need to turn to supplements. I see those cases as a last resort. Here’s another example from my own life: For years, I’ve been “borderline anemic.” That’s in spite of the fact that I eat red meat, egg yolks, dark leafy greens, dried fruit, beans and lentils. It only got worse after I had surgery a year ago and lost a lot of blood, which was not replaced with a transfusion (my doctor said I was in such great health that it was better for my body to synthesize new blood cells itself). I’ve been very mindful of incorporating those iron-rich foods into my diet, but I can only eat so much food! And I can’t eat only those foods (I’d get sick of them, and I’d develop other deficiencies). My iron stores were really low when I had them tested two months post surgery (shocking!), and I suspect that I haven’t raised them up high enough. That may be the reason for the nagging fatigue I’ve been feeling for months. I’m having bloodwork done in a few weeks, and if I am still low in iron, I will talk to my doctor about taking supplements. I’m tired of being tired!
Eating the right nutritious foods can have a profound effect on your body and your health, but at the end of the day the amount of food you eat counts, too! Strive to get a variety of foods in your diet, and for those particular foods that have a nutrient you need more of, don’t add them in, swap them in. There was a good article in the New York Times yesterday about “Swapping Meat for Nuts to Lower Diabetes Risk.” Notice the word “swapping”? It’s key!
If you’d like to read more about getting nutrients from food, I dug up a Harvard Medical School article from 2009, “Getting your vitamins and minerals through diet.” There are annoying popups to try to get you to subscribe to their newsletter, but the article is worth the annoyance. They even mapped out a 1200-calorie diet that meets or exceeds the Daily Recommended Intakes (DRIs) for all nutrients except vitamin D. There’s also a list of foods that provide a lot of nutrients relative to their number of calories. And I eat every single food on that list, thank you very much.